1. Technical Field of the Invention
The present invention relates to a power converter, and particularly to a synchronous rectifier having voltage limiting circuitry.
2. Background and Objects of the Invention
Power converters are known to convert an input voltage waveform having a DC component into a DC voltage waveform. Power converters employing an isolation transformer require employment of a rectification circuit to perform waveform conversion. Traditionally, rectifier circuits utilize diodes connected to the secondary side of the isolation transformer which conduct current only when forward biased. Synchronous rectifiers instead utilize transistor switches which are selectively turned on and off in synchronicity with the signal to be rectified in order to control conduction of current from the isolation transformer to the rectifier output.
The advancement in computational speeds and densities of integrated circuitry and corresponding systems has led to a reduction in the operating voltages thereof. Power supplies are accordingly expected to provide increased current at lower voltages while occupying less area. Because power loss through the diodes or transistor switches within a rectifier may account for nearly 40-50% of the total power loss in the power supply, employment of diodes having a low forward bias and/or transistor switches having low on-resistances are most desirous.
Large voltage spikes and high frequency ringing are typically generated within a rectifier during fast switching transitions, primarily due to the presence of parasitic inductances in the transformer and parasitic capacitances in the transistor switches. As a result, higher rated devices are needed to prevent breakdown of the diodes and/or transistor switches. Because use of the higher rated devices increases the power loss, such use to suitably handle the excessive voltage spikes and high frequency ringing conflicts with the need for employing diodes and transistor switches to suitably maintain low power loss.
In response to this situation, attempts have been previously made to suppress the high voltage spikes and high frequency ringing using additional circuitry. For instance, RC snubber circuits have been utilized which somewhat reduce peak voltages and dampening oscillations. However, the snubber circuits (FIG. 1) dissipate a noticeable amount of power and peak voltages within the rectifier nonetheless remain. Active clamping circuits (FIG. 2) have been previously proposed to suppress the voltage spikes. The active clamping circuits, however, substantially increase the complexity and component count of the rectifier. Consequently, there exists a need for a rectifier having reduced voltage spikes and high frequency ringing.
It is an object of the present invention to provide a rectifier circuit for use in a power supply which substantially eliminates voltage spikes and high frequency ringing.
It is another object of the present invention to provide a rectifier circuit having reduced power loss.
Another object of the present invention is to provide a rectifier circuit which minimally increases component count and power consumption of the rectifier.
Still another object of the present invention is to provide a rectifier circuit for a power supply which is simple in design and inexpensive to manufacture.